1. Field of the Invention
The present invention relates in general to the extraction of chrominance shape information for an interlaced scan type image, and more particularly to a method and apparatus for extracting the chrominance shape information for the interlaced scan type image, in which chrominance and luminance signals are matched with each other in consideration of a characteristic of the interlaced scan type image, thereby overcoming a color bleeding phenomenon to enhance a subjective picture quality.
2. Description of the Prior Art
Generally, video signal coding methods may be classified into a frame-based coding method which encodes the entire rectangular frame or picture and an object-based coding method which encodes only an arbitrary shaped region. The representative examples of the object-based coding method may be standards such as ISO/IEC JTC1/SC29/WG11 MPEG-4, ISO/IEC JTC1/SC29/WG1 JPEG2000, etc.
The object-based coding method using shape information is adapted to extract only specific regions, or objects, desired by the user from the entire image sequence by virtue of a segmentation technique, code the extracted regions and reconstruct the coded regions in a frame in a predetermined order. The shape information is used in the object-based coding method to make a distinction between the extracted objects. For example, the shape information can be used to classify an image into an object region and a non-object region (background). Accordingly, the shape information allows a coder and decoder to effect a signal process based on the object region rather than the entire image. The shape information may be either binary information or gray scale information.
The binary shape information is used to make a distinction between two objects in one image sequence, and the gray scale shape information is used to make a distinction among a plurality of objects in one image sequence. The binary shape information is represented by two values—for example, 0 and 1, or 0 and 255, etc., and the gray scale shape information is represented by any value within a predetermined range—for example, from 0 to 255.
On the other hand, for an application field requiring a high picture quality, such as a TV program broadcast, MPEG-4 supports both progressive and interlaced scan type images, similarly to MPEG-2.
Video scanning methods will hereinafter be described briefly. The video scanning methods may greatly be classified into a progressive scan type as shown in FIG. 1a and an interlaced scan type as shown in FIG. 1b. In the progressive scanning method of FIG. 1a, a one-frame image is composed of lines which are sequentially scanned. However, in the interlaced scanning method of FIG. 1b, a one-frame image is composed of lines which are alternately scanned. Namely, in the interlaced scanning method, a one-frame image consists of two field images sampled at different times, or a top field image and a bottom field image. Such a difference between the frame image and the field images requires that a video signal should be coded in a field unit as well as in a frame unit for the efficient coding of the field images.
When an object-based video process is performed, instead of generating or transmitting chrominance shape information of an object video signal, chrominance shape information is extracted by sub-sampling luminance shape information of the video signal in consideration of a sampling frequency ratio of luminance signal Y to chrominance signals Cb and Cr of chrominance signals. For example, considering the object-based vide codec, only the luminance shape information is inputted and transmitted to indicate a region in a frame and the chrominance shape information is extracted on the basis of the luminance shape information by virtue of an appropriate sub-sampling technique.
A conventional chrominance shape information extraction method will hereinafter be described in more detail. In the most video coding standards, a sampling frequency ratio of luminance signal Y to chrominance signals Cb and Cr is 4:2:0, as shown in FIG. 2. According to a 4:2:0 video format, the total number of chrominance pixels is ¼ that of luminance pixels (because the amount of chrominance data is reduced by ½, respectively, in horizontal and vertical directions).
In other words, one chrominance component (i.e., Cb or Cr) is present with respect to four luminance components. This luminance-to-chrominance ratio must identically be applied to the extraction of chrominance shape information used in coding the chrominance signals. In the current MPEG-4 video standard, a conservative chrominance shape sub-sampling method is used to extract chrominance shape information as shown in FIG. 4 with respect to luminance shape information as shown in FIG. 3 regardless of a video scanning type.
In a conventional conservative sub-sampling method, as shown in FIG. 4, four adjacent luminance shape pixels (also designated as alpha pixel) surrounding a chrominance pixel corresponding to chrominance shape information to be extracted and nearest to the chrominance shape pixel are set to a sub-sampling unit. If all of the four luminance shape pixels are background pixels as in sub-sampling units 1 and 4, the corresponding chrominance shape pixel is determined as background shape pixel. In the case where at least one of the four luminance shape pixels is an object pixel as in sub-sampling units 2 and 3, the corresponding chrominance shape pixel is determined as object shape pixel. FIG. 5a shows an arrangement of chrominance shape information extracted with respect to the luminance shape information of FIG. 3 by the conventional conservative sub-sampling method, where “□” indicates background shape pixel and “▪” indicates object shape pixel.
However, if a characteristic of an interlaced scan type image is not considered in the above-mentioned conservative sub-sampling method, a color bleeding phenomenon will occur.
This color bleeding phenomenon appears when any one of a luminance component and a chrominance component corresponding thereto is not present, namely, when no chrominance signal is present whereas a luminance signal is present, and vice versa.
FIG. 6 shows color bleeding portions appearing when the result extracted by the conventional conservative sub-sampling method in FIG. 4 is projected on a screen in an interlaced scanning manner. As shown in this drawing, in the case where the conservative sub-sampling method for the progressive scan type image is applied to an interlaced scan type image, a color bleeding phenomenon occurs in which there are present portions 91, 92 and 93 with luminance components and no chrominance component corresponding thereto and a portion 94 with a chrominance component and no luminance component corresponding thereto.
Such a sampling error of chrominance shape information, in turn, results in the occurrence of an error in extracting the chrominance shape information. As a result, a chrominance signal of a background, not intended to be expressed, may be processed as that of an object. This has no effect on an objective picture quality, but it results in a significant degradation in subjective picture quality because the background signal is shown in the object. The picture quality of a video signal may be estimated in a quantitative manner based on a signal-to-noise ratio (SNR) or in a qualitative manner based on human visual sense. The former is called the objective picture quality and the latter is called the subjective picture quality. The subjective picture quality is estimated qualitatively according to a difference between human visual impressions on image regions in a frame. Hence, the subjective picture quality cannot be estimated in such a quantitative manner as the SNR, but it is as important as the objective picture quality in estimating the performance of video processing techniques or units. In this connection, in the object-based video processing desiring excellent picture quality, a method is keenly required which is capable of extracting accurate chrominance shape information.